1. Field of the Invention
The invention is based on a fuel injection system for an internal combustion engine and particularly to such a system in which fuel may be injected at two differently high fuel pressures.
2. Description of the Prior Art
One injection system of the type with which this invention is concerned has been disclosed by European Patent Disclosure EP 0 711 914 A1, for instance.
For better comprehension of the ensuing description, several terms will first be defined in more detail: In a pressure-controlled fuel injection system, a valve body (such as a nozzle needle) is opened counter to the action of a closing force by the fuel pressure prevailing in the nozzle chamber of an injector, and thus the injection opening is uncovered for an injection of the fuel. The pressure at which fuel emerges from the nozzle chamber into the cylinder is called the injection pressure. The term stroke-controlled fuel injection system is understood in the context of the invention to mean that the opening and closing of the injection opening of an injector takes place with the aid of a displaceable valve member on the basis of the hydraulic cooperation of the fuel pressures in a nozzle chamber and in a control chamber. An arrangement is furthermore described below as central when it is provided jointly for all the cylinders, and as local if it is intended for only a single cylinder.
In the pressure-controlled fuel injection system known from EP 0 711 914 A1, with the aid of a high-pressure pump, fuel is compressed to a first, high fuel pressure of about 1200 bar and stored in a first pressure reservoir. The fuel that is at high pressure is also pumped into a second pressure reservoir, in which by regulation of its fuel delivery using a 2/2-way valve, a second high fuel pressure of about 400 bar is maintained. Via a central valve control unit and a central distributor device, either the lower or the higher fuel pressure is carried into the nozzle chamber of an injector. There, by means of the pressure, a spring- loaded valve body is lifted from its valve seat, so that fuel can emerge from the nozzle opening.
In this known fuel injection system, the fuel for an injection from the applicable central pressure reservoir is split via the central valve unit and the central distributor device to the individual injectors. The maximum possible injection window in each case is determined jointly by the valve unit and the distributor device.
To enlarge the injection window for the lower fuel pressure, that is for the pre-injection or the post-injection, according to the invention the lower fuel pressure is metered directly from the central pressure reservoir, without proceeding by way of the distributor device.